Edible oils can be obtained from a number of different vegetable raw materials from which the oils are derived. The oils may be extracted using an organic solvent like hexane or they may be derived from the vegetable crops by mechanical methods such as hydraulic pressing (Anderson (2005)).
The crude vegetable oils from such processes will besides glycerides, i.e. fatty acid esters of glycerol, contain a number of other components of which lecithins (phosphatides) and free fatty acids typically are the most predominant ones. Especially the free fatty acids may constitute a significant portion of a crude vegetable oil and are perceived as undesirable components of the vegetable oil, as they affect its organoleptic properties. The free fatty acids are formed by hydrolysis of triglyceride in reactions taking place due to damages of the crop during harvesting and/or storage of the seeds or fruits before the extraction of the oils take place.
For an oil of a good quality it is generally required that those minor components are reduced to a low concentration to provide a vegetable oil product having a bland taste and an acceptable smell. Codex Alimentarius (Codex Alimentarius; Fats, Oils and Related Products, vol. 8, 2. ed. FAO/WHO Rome 1993) generally recommends an acid value of maximum 0.6 equivalent to approx. 0.3% free fatty acids for edible oils.
To meet the demand for high quality and agreeable taste the crude vegetable oils are typically refined by chemical and/or physical processes to remove a number of minor constituents present in the crude oil.
As described by Anderson (2005), a refining process typically includes a degumming step where phosphoric acid is added to the oil, thereby rendering the phospholipids of the oil soluble in water. The water may be removed from the refined vegetable oil by gravity techniques.
Next step is normally the removal of the free fatty acids, also referred to as the deacidification. The deacidification may be performed by the alkaline wash process, which involves the addition of aqueous, diluted lye to the vegetable oil. The lye converts the free fatty acids into the corresponding soaps, which are soluble in water, and which can be removed in a separator followed by a series of washing step with water to provide an acceptable removal of traces of soaps. The alkaline wash-process requires a number of washing step, which consume a significant amount of energy and makes the process complicated and expensive.
Next step of the refining process is typically the bleaching of the degummed, deacidified, vegetable oil. The bleaching may involve addition of bleaching clay like bentonite or silicon dioxide to remove colour components as well as the traces of free fatty acids from the vegetable oil. The addition of the clays often takes place in closed tanks under reduced pressure and after a specified duration the oil is filtered to provide an edible quality oil.
Instead of the above-mentioned alkaline wash process the free fatty acids may be removed by another process. Here the lye addition step is omitted and the bleached oil, now with a high content of free fatty acids, is treated in a steam distillation process known as deodorisation, see e.g. Anderson (2005). In this process the vegetable oil is heated to a high temperature under vacuum. The heating is performed by contacting the vegetable oil directly with superheating steam under conditions which allows for a good contact between the oil and the steam, and thus an efficient distillation. The steam distillation will remove the low boiling components, in this case the free fatty acids, and a number of colouring products as well as off flavour components, thereby yielding a bland and stable vegetable oil. In the case of alkaline washed vegetable oil, a deodorisation will improve the quality by removing the last quantities of free fatty acids. The deodorisation may be implemented as a batch operation in large tanks or it may be implemented continuously in columns equipped with trays or other installations, which provide good contact between steam and oil. An example of a deoderisation process is described in WO 98/18888.
Conventional refining of vegetable oil makes use of high temperature deodorisation (>200 degrees C.) over extended periods of time, which may damage the temperature labile components of the vegetable oil. Additionally, it is well known that the deodorisation can cause the formation of trans fatty acids by thermal rearrangement of unsaturated bonds from the natural occurrence of cis to trans isomers (Harper (2001) and Greyt et al (2005)), and the latter is associated with health risks like heart infarctions.